The present invention relates generally to a method of and system for monitoring a plasma etch process for processing a substrate such as a semiconductor device. Plasma etch processes have been used in the semiconductor industry since the 1970's. Generally, a plasma etch process is used to selectively "etch" or remove material from a semiconductor wafer thereby forming a desired microelectronic pattern.
In a plasma etch system, such as a dry etch system for processing semiconductor wafers, processing gas is introduced into a plasma processing chamber (hereinafter simply referred to as a chamber) in a vacuum environment to generate a plasma. Highly chemically reactive ions or radicals are produced by the ionization or the dissociation of the processing gas. Those active particles physically or chemically act to etch off a desired area of a film on a semiconductor substrate to form a microelectronic pattern. For example, in the method and apparatus disclosed in Gabriel et al., U.S. Pat. No. 4,954,212, the plasma etches the top layer of the wafer only when the power supply is activated and when the power reflected by the chamber is relatively low. In U.S. Pat. No. 4,954,212, activating the power supply "strikes" the plasma, and activates the etching process.
In general, while etching any particular layer, plasma light emissions are generated at frequencies corresponding to the chemical makeup of the layer being etched and the etching gas chemistry used. In other words, the layer being etched chemically combines with the plasma, creating predictable chemical compounds, and the plasma light emission frequencies present in the plasma correspond to these chemical compounds.
In many plasma etch processes these frequencies are detected using a light or optical sensor. Typically, the optical sensor is set up, using narrow bandpass filters, to monitor the intensity of light at a frequency associated with the layer being etched. For example, in some etch processes, when the measured intensity falls below a specified threshold, a sensor generates an endpoint signal that is usually transmitted to a controller, which turns off a power supply and thereby stops the etch process.
A plasma light emission spectrometer has been widely used because it can monitor many factors which correspond to an etching characteristic. Perhaps the most important signal to monitor is the endpoint. In other words, determining when to stop the etching process after the layer being etched is removed but before the next layer is destroyed. For example, in U.S. Pat. No. 4,954,212, an endpoint signal is defined as one that either falls or rises at the end of a plasma etch process. Light intensity based endpoint signals that rise at endpoint are typically related to the concentration of reactants in the chamber or the concentration of products or reactions involving the underlying material, while endpoint signals that fall at endpoint are typically related to the concentration of reactive intermediates or products or reactions involving the material being etched.
Due to the extreme environmental conditions within a chamber, the sensor device is usually placed outside the chamber. This configuration requires that the chamber have an emission detection window for the plasma light emissions to pass through before detection by a spectrometer or other sensor device. The detection window is usually made of quartz glass and typically has a diameter of several centimeters.
A major problem encountered during a plasma etch process is the build up of contaminants on the surface of the detection window. This is due to a reaction product which is deposited on the surface of the window, or an unevenness which is created thereon by the impact of the ions from the plasma. As a result, when a plasma light emission passes through the window, a waveform component inherent to the material forming the deposit is absorbed or it is randomly reflected on the inner surface of the window so that the light intensity is decreased. This is a major problem due to the importance of detecting small changes in the intensity of the plasma light emissions. For example, determining the endpoint requires detecting minuscule changes in the plasma light emission intensity. These changes are difficult or impossible to detect when the detection window is coated with contaminants. This problem results in false readings of the state of the etch process, resulting in defective products.
Most of the prior art solutions to this problem involved apparatus and methods for cleaning the deposits from the detection window. Generally, the windows were cleaned following each etch process or when product failures indicated that the window contaminants were sufficient to cause erroneous plasma light emission readings. Usually, the cleaning process required opening the chamber or disrupting manufacturing activities by performing an in-situ cleaning procedure. In addition to exposing sensitive and expensive equipment to potentially frequent accidental damage, this approach consumed valuable time that could otherwise be used for manufacturing production.
Koshimizu, U.S. Pat. No. 5,322,590, discloses a proposed solution to the cleaning problem discussed above. In U.S. Pat. No. 5,322,590, a heating device is proposed for removing or preventing the build-up of contaminants on the window without disrupting the etch process. As discussed in this patent, contaminants are likely to adhere to the coolest portion of the interior of the chamber. Typically, the coolest portion is the detection window. Therefore, a ring-shaped heater is mounted on the outer surface of the observation window. A thermometer is positioned near the heater and detects the window temperature. The thermometer outputs a signal representing the window temperature and a controller controls the power supply attached to the heater. During an etch process, the window is maintained at a predetermined temperature to prevent contaminant deposition. Alternatively, the heater is used to remove the contaminants at the end of an etch process.
Unfortunately, adding these sensitive measurement and heating devices in the hostile plasma etch process environment may contribute to frequent and costly maintenance, and a potential risk for fires. Furthermore, this approach may not be effective for all processing conditions and materials.
Another prior art method and apparatus proposed to overcome the detection inaccuracies resulting from contaminants on the observation window are discussed in Imatake, et al., U.S. Pat. No. 5,759,424. In U.S. Pat. No. 5,759,424, the plasma light emission generated in the chamber, and a light having a known spectrum emitted from a light source external to the chamber and transmitted through the interior of the chamber, are detected. Next, the difference between the light emission of the plasma and the light of the known spectrum is determined. The internal state of the chamber is determined based on the difference between the spectrums. In the preferred embodiment of U.S. Pat. No. 5,759,424, a reference light illumination means is positioned external to the processing chamber and projects a reference light to a plasma light emission monitoring means, via one of a pair of opposing windows to the plasma light emission monitoring means located on the other side of the second window.
In other words, a separate light (reference light) is external to the processing chamber and it is transmitted through the processing chamber via one of a pair of opposing windows and emitted out of the processing chamber via the other window. The reference light is detected by a spectrometer and compared with an original source light (which is not transmitted through the processing chamber) so that the transmissivity for each frequency is determined. Next, the reference light is turned off and the spectrum of the plasma light emission is detected. The spectrum is divided by the transmissivity determined for each frequency by using the reference light so that the detection error due to the affect of the window is compensated and the plasma light emission thereby determined.
Again, the additional sensitive equipment required in this invention may be a problem from a maintenance standpoint because sensitive detection equipment is prone to damage due to the hazards inherent in a chamber environment. Further, the need for a dual windowed chamber may require costly modifications to existing chambers and chamber manufacturing methods. Therefore, the need for an easy, low maintenance and cost effective solution is still apparent.
It is an object of the present invention to overcome the shortcomings of the prior art by providing a simple, efficient and inexpensive method and apparatus for monitoring a plasma etch process without being affected by the change in a plasma light emission transmission characteristic caused by contaminants on a detection window.